Why is it expected to be on one’s own by a certain age? Because your parents would like to have their lives back.
My son is my only child; he is 22 and he lives with my fiancé and I. We have the home arranged so that he has plenty of his own space. He and I have a very close relationship, and he gets along very well with my fiancé. He pays a little rent and has his own car loan and insurance and pays his own bills. His girlfriend stayed here most nights and we got along well. In fact, they broke up not too long ago, but she got together with us for dinner last week because she misses us.
Despite how good things are, I look forward to him moving out one day, and us having our home to ourselves. I raised him to be responsible and independent, but this economy makes it very difficult for people to live on their own. So I accept that this is the best arrangement for him, and I’m glad to be able to help him out. That arrangement is between the parties involved to communicate about what works best for them.
As for the expectations of others in society, that’s nonsense to which I’ve never paid any attention. Life is complex and challenging, and no one can judge you as long as you are not hurting others or mooching from others.